Already dealing with parched conditions, the U.S. Southwest faces the threat of megadroughts this century as temperatures rise, says a new study that found the risk is reduced if heat-trapping gases are curbed.

Oppressive dry spells lasting at least two decades have gripped the Southwest before, but scientists said future megadroughts would be hotter and more severe, putting a strain on water resources.

The study, published in the journal Science Advances, is the latest to find that droughts more extreme than what is currently being experienced could become more common as the planet warms.

Irrigation pipes sit along a dry irrigation canal on a field farmed by Gino Celli, who relies on senior water rights to water his crops, near Stockton, Calif. New research published on Wednesday, Oct. 5, 2016 found that increasing temperatures will greatly increase the risk of megadroughts in the Southwest region of the U.S.

WHAT IS A MEGADROUGHT?

Long-lasting mega-droughts could occur with increasing frequency in the western US later this century if no action is taken to rein in climate change, researchers said.

Mega-drought is defined as any drought as bad as the worst already seen in the 20th century, but lasting 35 years or more.

Onje study by Cornell University is the first to predict that the coming intense dry spells could exceed the decades-long mega-droughts that occurred centuries ago and are blamed for the demise of certain civilisations in 13th century.

Using computer modeling, researchers calculated there's between a 70 percent and 90 percent chance the Southwest will experience a megadrought later this century.

If precipitation is below normal, the risk jumps to 99 percent — 'virtually certain,' said lead researcher Toby Ault of Cornell University.

'Megadroughts are comparable in severity to the worst droughts of the 20th century but are of much longer duration, ' the team warn.

'A megadrought in the American Southwest would impose unprecedented stress on the limited water resources of the area, making it critical to evaluate future risks not only under different climate change mitigation scenarios but also for different aspects of regional hydroclimate.'

'We find that regional temperature increases alone push megadrought risk above 70, 90, or 99% by the end of the century, even if precipitation increases moderately, does not change, or decreases, respectively.

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US Southwest is 'virtually certain' to suffer a megadrought this century, researchers warn. Using computer modeling, researchers calculated there's between a 70 percent and 90 percent chance the Southwest will experience a megadrought later this century.

If countries follow through with the Paris climate agreement to reduce emissions from the burning of coal, natural gas and oil to limit global temperature rise to well below 2 degrees Celsius (3.6 degrees Fahrenheit), then the risks are cut nearly in half, according to calculations.

'The likelihood of a megadrought is already increasing, and that risk will continue to go up as long as temperatures keep rising,' Ault said in an email.

The Colorado River basin, which spans parts of Arizona, California, Colorado, New Mexico, Nevada, Utah, Wyoming and Mexico, has been in the midst of a historic drought for the past 16 years.

Lake Mead is now surrounded by a distinctive white mineral 'bathtub ring' showing the 130-foot drop in water levels since the beginning of the regional drought.

California, which is entering a sixth year of exceptional dryness, imposed mandatory statewide cuts on water usage last year that left brown lawns and fallowed fields. Restrictions have since eased with local water districts taking control of conservation efforts.

THE 'BATHRUB RING' THAT REVEALS LAKE MEAD'S 130FT DROP

The Colorado River basin, which spans parts of Arizona, California, Colorado, New Mexico, Nevada, Utah, Wyoming and Mexico, has been in the midst of a historic drought for the past 16 years.

Lake Mead is now surrounded by a distinctive white mineral 'bathtub ring' showing the 130-foot drop in water levels since the beginning of the regional drought.

A riverboat glides through Lake Mead on the Colorado River at Hoover Dam near Boulder City, Nev. Amid an historic drought in the West, federal water managers are due to release an annual projection of surface levels at Lake Mead that'll determine whether water deliveries from the crucial Colorado River reservoir will be cut next year to Arizona, Nevada and California.

A federal report earlier this year shows the surface level of the lake behind Hoover Dam is expected to remain high enough this year to avoid a shortage declaration in 2017. But it'll still be a mere 4 feet above a 1,075-foot elevation action point.

For 2018, the U.S. Bureau of Reclamation projects the lake level could fall short — by less than 1 foot.

A 'bathtub ring' surrounds Lake Mead near Hoover Dam, which impounds the Colorado River at the Arizona-Nevada border, on March 30, 2016. The white ring shows the effects of a drought which has caused the level of the lake to drop to an historic low. The ring is white because of the minerals which were deposited on the previously submerged surfaces.

On Wednesday, state regulators said some water districts are not conserving enough.

The new forecasts of a future megadrought are 'scary for sure,' said Jonathan Overpeck, a University of Arizona climate scientist who wasn't involved in the research.

Based on studies of tree rings and other data, past megadroughts in the Southwest have lasted between 20 to 35 years, depending on the location.

'It is important to realize that future droughts will be much hotter, and thus the impacts of these droughts will be much more severe,' Overpeck said in an email.